This invention relates to a solid electrolyte capacitor having a metallic cathode collector layer in direct contact with a manganese dioxide electrolyte layer and a method of producing the same.
A solid electrolyte capacitor of a well known type employs a valve metal typified by aluminum and tantalum as the material of the anode and manganese dioxide as the solid electrolyte. The anode takes the form of a porous mass or body obtained by pressing and sintering a valve metal powder. A dielectric layer is formed on the surface of the anode body by a well known anodic oxidation technique using an acid solution as an electrolyte. A solid electrolyte layer is formed on the dielectric oxide layer by immersing the anode body in an aqueous solution of manganese nitrate and heating the wet anode body to cause pyrolytic decomposition of manganese nitrate to manganese dioxide. The immersion and heating are repeated several times until a satisfactorily dense and stable manganese dioxide layer is formed. Then a contact layer is formed on the solid electrolyte layer by the application of, for example, colloidal graphite in the form of an aqueous dispersion, followed by drying. A metallic cathode collector layer is formed on the contact layer by the application of conductive paste containing dispersed therein fine particles of, for example, silver.
The manganese dioxide layer formed by the pyrolysis of a manganese nitrate solution is a spongy one and, when microscopically observed, has an uneven and irregularly rugged surface. Accordingly it is practically impossible to form a cathode collector layer directly on the manganese dioxide layer with a sufficiently low electrical resistance. The provision of the graphite contact layer between the manganese dioxide layer and the cathode collector layer is for the purpose of solving this problem. However, the contact layer itself is not easy to form satisfactorily. To realize a full impregnation of the irregularly rugged surface region of the manganese dioxide layer with graphite, it is necessary to use a uniform dispersion of extremely fine, for example smaller than 1 .mu.m, graphite particles such as "AQUADAG" (trademark of Acheson Colloid Co.) which is an ammoniacal aqueous dispersion of colloidal graphite. Such a graphite dispersion is diluted to a suitable concentration to immerse the anode body therein. The contact graphite layer is completed by drying the graphite-impregnated anode body at 80.degree. - 100.degree. C.
Thus the production of a conventional solid electrolyte capacitor of the described type needs a large number of steps for providing a cathode collector layer. Besides, the inherent surface ruggedness of the manganese dioxide layer necessitates the application of a costly silver paint in a considerably large quantity. Apart from the troubles at the formation of the contact and collector layers, the insufficient denseness and the rugged surface of the manganese dioxide layer cause that the capacitor suffers from a considerably large loss factor and unsatisfactory frequency and/or impedance characteristics.
As to the physical structure of the manganese dioxide layer formed by the pyrolysis of a manganese nitrate solution in the production of solid electrolyte capacitor, a remarkably improved method has recently been developed by A. Nishino et al of Matsushita Electric Industrial Co. and is disclosed in U.S. Patent application Ser. No. 632,079 filed Nov. 14, 1975, now U.S. Pat. No. 4,038,159. Among the coinventors of the present invention, A. Nishino and H. Hayakawa are included in the coinventors in the above referred prior application.